Waller County, Texas, has had a complicated racial history since the days when it was a part of Mexico. At one of its first settlements, Bernardo Plantation, about 100 slaves grew cotton on a large farm on the banks of the Brazos. Yet in the years before Texas fought Mexico for its independence, the area became a magnet for free blacks from elsewhere in the South who sought a welcoming home.

The messy, confusing double legacy of that history has persisted to the present, most recently...